1. Field of the Invention
The present invention relates to a head unit, and more particularly to a head unit having a recording/reproducing head which is swingably supported by a head supporting means such as a resilient plate in such a way that the recording/reproducing head makes contact with a recording medium.
2. Description of the Prior Art
As an example of a recording/reproducing apparatus using a head unit of this type, known is a magnetic disc apparatus which records or reproduces information on or from a magnetic disc as a magnetic recording medium.
FIG. 1 shows an example of an arrangement of this type of magnetic disc apparatus.
In FIG. 1, reference numeral 1 denotes the entire magnetic head unit. The lower part of a supporting member 2 which supports the whole unit 1 has an arm 3B as an integral part thereof. A magnetic head 4B is mounted on the top of the end of the arm 3B. A resilient supporting plate 5 is fixed to the top surface of the supporting member 2. This supporting plate 5 supports the upper arm 3A in a position opposite to that of the lower arm 3B. On the bottom surface of an end of the arm 3A is mounted a magnetic head 4A.
The force of a coil spring 7 disposed between the front end of a bracket 6 whose rear end is fixed to the top of the supporting member 2 and the rear end of the arm 3A causes the upper arm 3A to be pressed toward the lower arm 3B. This pressure causes the upper magnetic head 4A to press against the lower magnetic head 4B via the disc 8 which is the magnetic recording medium, thereby holding the disc 8 between the heads 4A and 4B.
When recording or reproducing, the disc 8 is rotated by a motor (not shown) in a direction indicated by an arrow W around a central axis shown by a dash and dotted line in the drawing. The magnetic heads 4A and 4B make contact respectively with the upper and lower surfaces of the disc 8, and then recording or reproducing is performed.
In order that this recording and reproducing be performed stably and accurately, the contact surfaces of the magnetic heads 4A and 4B must follow any displacement in the surface of the disc 8, and it is necessary that the magnetic heads 4A and 4B always make contact uniformly and in parallel with the surface of the disc 8. To achieve such contact, the magnetic heads 4A and 4B are supported by a resilient plate so that the heads can be displaced within a predetermined range.
More specifically, as shown in FIG. 2, the magnetic head 4A is mounted on a resilient plate 9 which is bridged over an opening at the end of the arm 3A which project downwards. The magnetic head 4A is supported by the resilient plate 9 so that the head 4A can be displaced within a predetermined range by the resilient plate 9.
The other magnetic head 4B is also attached to and supported by a member similar to the resilient plate 9 with upside-down relation.
The resilient plate 9 is called a gimbal spring. It is formed into the shape as shown in FIG. 3 from a thin sheet having resiliency such as a resilient metal. As shown in FIG. 3, the resilient plate 9 is shaped roughly to be rectangular. From inside this rectangular shape, two pairs of elongated holes 9a and 9a, and 9b and 9b are formed in such a two-fold cutting way that each hole is substantially U-shaped and one hole of one pair is opposite to the other of the pair with a small gap therebetween, with each of the inner and outer pairs being disposed with an angular displacement of 90 degrees. That is, by this cutting, a roughly rectangular shaped frame 9c is formed on the inside of the elongated holes 9a and 9a, and a roughly rectangular shaped mounting portion 9e is formed on the inside of the elongated holes 9b and 9b. The frame 9c is linked to the peripheral portion 9g of the resilient plate 9 by two bridge portions 9d and 9d which are substantially in the center on both the right- and left-hand sides of the peripheral portion 9g, as shown in FIG. 3. The mounting portion 9e is linked to the frame 9c by two bridge portions 9f and 9f (the bridge 9f on the rear side is not seen) which are substantially in the center on the back and the front sides of the frame 9c, as shown in FIG. 3.
This arrangement allows the mounting portion 9e of the resilient plate 9 to swivel freely around an X axis passing through the bridge portions 9d and 9d and around a Y axis passing through the bride portions 9f and 9f, within a range permitted by the lengths and degree of resiliency of the bridge portions 9d and 9d, and 9f and 9f. That is, the arrangement permits free swivelling in the directions indicated by arrows C and D around a center A (as shown in FIG. 2) of swivelling which is at an intersection of the axes X and Y.
A core portion 10 in the magnetic head 4A is attached to the mounting portion 9e through a hole (not shown) formed in the mounting portion 9e and fixed in place as shown in FIG. 2. This core portion 10 consists of two thin sheet shaped cores and a spacer therebetween. Mask blocks 11 and 12 are fixed to the lower surface of the mounting portion 9e so as to support both sides of the lower half part of the core portion 10 which protrudes below the mounting portion 9e.
The mask blocks 11 and 12 are to support and protect the core portion 10. The former is substantially rectangular in shape, and the latter is formed into the shape of a block having a substantially U-shaped cross-section. The bottom surfaces of the mask blocks 11 and 12 and the bottom surface of the core portion 10 forms the contact surface 13 which makes uniform contact with the magnetic disc 8.
A magnetic gap (not shown) which performs recording and reproducing is formed in the core portion 10 on the contact surface 13.
Furthermore, a coil 14 for electro-magnetic conversion is wound arround the upper half part of the core portion 10 protruding from the top side of the mounting portion 9e. The coil 14 is connected by lead wires 16 to a flexible printed circuit board 15 having a top end which is fixed to one edge of the mounting portion 9e.
The magnetic head 4A attached to the mounting portion 9e of the resilient plate 9 which can swivel freely through a predetermined range is pressed at one point by a pivot 17 as a pressing member, as shown in FIG. 2. The rear end of the pivot 17 is fixed in position to the top part (not shown) of the arm 3A. The pivot 17 is positioned so that its tapered end presses against the pressure point B corresponding to the center of swivelling A on the mounting portion 9e.
As described above, the magnetic head 4A is swivelably mounted on the mounting portion 9e of the resilient plate 9, and arranged so that the center of swivelling A of the mounting portion 9e coincides with the pressure point B. Therefore, even an extremely small force will easily cause the head 4A to swivel in the directions shown by the arrows C and D in FIG. 3. For example, when the head 4A swivels in the direction of arrow C in FIG. 3 to incline the head 4A, the head 4A assumes an inclined position as shown in FIG. 4. A dash and dotted line indicated by the symbol V in FIG. 4 is a vertical line. Thus, the magnetic head 4A is swivelled by even a very small force, and can always follow the displacement of the disc 8 during recording and reproducing.
In the above arrangement, however, if any disequilibrium between the pressure of the contact surface 13 of the magnetic head 4A and the sliding resistance of the disc 8 should occur during recording or reproducing, so that oscillations accompanied by resonance arise in the magnetic head 4A, recording of information on to the magnetic disc 8 by the magnetic head 4A, and reproducing of information read from the magnetic disc 8 by the magnetic head 4A will be unstable.
It is supposed that these oscillations are caused by resonance which is produced when there is a slight deviation in the alignment between the center of swivelling A and the pressure point B of the mounting portion 9e on the resilient plate 9, since the magnetic head 4A is able to swivel even by a very small force.
This defects, of course, arises also with the magnetic head 4B.
Further, the above defects are not solely related to magnetic head units, but also occur in a head unit of any type of a recording/reproducing apparatus such as optical disc apparatus which perform recording and/or reproducing of information by making contact with an optical disc.